Adequate sexual function is a complex interaction of hormonal events and psychosocial relationships. There are four stages to sexual response as described in the International Journal of Gynecology & Obstetrics, 51(3):265–277 (1995). The first stage of sexual response is desire. The second stage of sexual response is arousal. Both physical and emotional stimulation may lead to breast and genital vasodilation and clitoral engorgement (vasocongestion). In the female, dilation and engorgement of the blood vessels in the labia and tissue surrounding the vagina produce the “orgasmic platform,” an area at the distal third of the vagina where blood becomes sequestered. Localized perivaginal swelling and vaginal lubrication make up the changes in this stage of sexual response. Subsequently, ballooning of the proximal portion of the vagina and elevation of the uterus occurs. In the male, vasodilation of the cavernosal arteries and closure of the venous channels that drain the penis produce an erection. The third stage of sexual response is orgasm, while the fourth stage is resolution. Interruption or absence of any of the stages of the sexual response cycle can result in sexual dysfunction. One study found that 35% of males and 42% of females reported some form of sexual dysfunction. Read et al, J. Public Health Med., 19(4):387–391 (1997).
While there are obvious differences in the sexual response between males and females, one common aspect of the sexual response is the erectile response. The erectile response in both males and females is the result of engorgement of the erectile tissues of the genitalia with blood which is caused by the relaxation of smooth muscles in the arteries serving the genitalia.
In both pre-menopausal and menopausal females, sexual dysfunction can include, for example, sexual pain disorders, sexual desire disorders, sexual arousal dysfunction, orgasmic dysfunction, dyspareunia, and vaginismus. Sexual dysfunction can be caused, for example, by pregnancy, menopause, cancer, pelvic surgery, chronic medical illness or medications.
In males, some pharmacological methods of treating sexual dysfunctions are available, however, such methods have not proven to be highly satisfactory or without potentially severe side-effects. Papaverine now widely used to treat impotence, is generally effective in cases where the dysfunction is psychogenic or neurogenic and where severe atherosclerosis is not involved. Injection of papaverine, a smooth muscle relaxant, or phenoxybenzamine, a non-specific antagonist and hypotensive, into corpus cavernosum has been found to cause an erection sufficient for vaginal penetration, however, these treatments are not without the serious and often painful side effect of priapism. Also, in cases where severe atherosclerosis is not a cause of the dysfunction, intracavernosal injection of phentolamine, an α-adrenergic antagonist, is used.
As an alternative or, in some cases an adjunct to phosphodiesterase inhibition or α-adrenergic blockade for the treatment of erectile dysfunction, prostaglandin E1 (PGE1) has been administered to the penis or by intracavernosal injection. Two products containing PGE1 have been approved by the U.S. Food and Drug Administration for the treatment of erectile dysfunction by intracavernosal injection (CAVERJECT®, Pharmacia & Upjohn Company, Kalamazoo, Mich.) and intraurethral administration (MUSE®, Vivus Incorporated, Mountain View, Calif.). A major side effect frequently associated with intracorpral or transurethral delivered PGE1 is penile pain and burning. In addition, priapism, infection, penile corporal fibrosis, fibrotic nodules, hypotension, bruising and hematomas may occur. Swelling and ulceration of the penile skin at the site of injection have also been reported.
There is a need in the art for the treatment of sexual dysfunctions, particularly treatments that do not have the undesirable side effects of those agents currently being used. The present invention is directed to these, as well as other, important ends.